The circulatory or cardiovascular system is composed of the heart, arteries, capillaries, venules and veins.

Artery

The arteries are strong, flexible blood vessels that carry blood away from the heart. Their strenght helps maintain blood pressure while the heart is between beats. Smaller arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease bloodflow to a particular area, depending on the body's needs.

Vein

The vessels that return blood to the heart.

Capillary

The smallest blood vessels in the circulatory system. Capillaries have thin walls that facilitate the transfer of oxygen and glucose into a cell and waste products such as carbon dioxide back out into the blood stream to be carried away and taken out of the body via the lungs. They act as the bridge between arteries, carrying blood away from the heart, and veins, carrying blood back to the heart.

Vasodilation

Excessive relaxation or dilation of the blood vessel walls. The resulting low blood pressure causes an inadequate blood supply to body cells, which can result in shock (blood pressure too low to sustain life). Causes of excessive dilation include head injury, liver failure, poisoning, drug overdoses and severe bacterial infection.